As a technique for reducing the sliding resistance of sliding members, hitherto, it has been known to coat the sliding surface of a sliding member with a solid lubricant such as molybdenum disulfide, tetrafluoroethylene resin, or tin oxide. Such solid lubricants are not only low in frictional resistance but are also relatively soft, and hence are easily worn in an initial phase of operation, and the so-called initial fitting is terminated at an early stage, and the sliding resistance and wear amount are stabilized at a low level. As the coating methods for solid lubricants, a method for dispersing and applying a solid lubricant on a binder resin, and a coating method by plating are known. Japanese Patent Application Laid-open No. 11-193455 discloses a technique of forming a molybdenum sulfide film on the sliding surface of a sliding member by sputtering.
However, in the method for applying a solid lubricant on the binder resin, the function of the solid lubricant is not exhibited sufficiently due to the presence of the binder portion (for example, about 50%). In addition, common problems in the prior art include changes in dimensions of sliding members by formation of solid lubricant film, difficulty in control of thickness of solid lubricant film, risk of peeling of solid lubricant film, and shortness of service life. In particular, the method for forming a solid lubricant film by sputtering is higher in equipment cost and manufacturing expenses, and it is difficult to treat curved surfaces.
Pistons for internal combustion engines (hereinafter called pistons) are designed to maintain their position by receiving a part of an explosion pressure in a combustion chamber by a piston ring sliding on a sleeve, and bringing a skirt portion provided at the lower side of the piston ring into contact with the sleeve. Therefore, if the skirt portion and sleeve do not contact with each other favorably, the sliding resistance increases and the fuel economy is sacrificed, or possibly due to abnormal contact between the piston and the sleeve, galling, seizure, or unusual sounds may occur.
Japanese Utility Model Application Laid-open No. 52-16451 and Japanese Utility Model Application Laid-open No. 57-193941 propose methods for forming fine dimples by sand blasting or shot peening in the skirt of the piston, but such methods are known not to be effective for preventing occurrence of unusual sounds.